This invention relates to magnetic-disc data storage devices having movable read-write heads.
In such devices data is arranged on the discs in tracks, and a single head is movable to access a number of tracks. During a transfer the head is positioned over a track and should be aligned with it accurately. This ensures, if the head is reproducing, that it receives a signal of the maximum strength free from interference from neighbouring tracks, and, if the head is recording, that the neighbouring tracks are not overwritten and the data is positioned in the expected place for a subsequent read operation. The more accurately the head can be positioned, the closer the tracks can be packed to one another on the disc.
When the storage device uses multi-disc packs with a head for each surface and the different heads coupled together it has been proposed to devote one surface to servo tracks which the head for that surface can be caused to follow. The remaining heads can thus be brought to reproducible positions on the discs. But this arrangement is wasteful of space that could be used to carry useful data, and is impractical for disc packs with only a few data surfaces.
An alternative is to position a head by reference to a separate position transducing system mounted in the device itself. A problem which then arises is that there may be temperature differences between the disc and the rest of the apparatus. The resultant differences in expansion may impair alignment between head and track.
Such variations occur particularly during the initial period after the storage device has been started from rest and while the various elements are reaching temperature equilibrium. The problem is especially acute if the discs are exchangeable, because a cold disc pack may be placed in a warm drive unit, or a warm disc pack in a cold drive unit. While equilibrium is being established the head may not be accurately positioned over the desired track, preventing reliable recording or reproduction of data.
It has been proposed to measure the temperatures of the disc and the position transducer and apply a correction to the positioning mechanism based on the predicted expansions. But the temperatures can be measured only indirectly (for the disc from the air flowing over them and for the position transducer as the temperature of the body of the drive unit) and without taking into account differences of temperature within the parts concerned. It is also difficult to predict accurately the total rates of expansion of the assemblages of which they are made up.